Display systems based upon the use of electrophoretic ink and magnetophoretic ink are a new category of display system that merge the attributes of conventional paper for conveying static images with the performance of conventional emissive displays for conveying dynamic images. In what follows, electrophoretic ink and magnetophoretic ink will be referred to collectively as “phoretic ink.”
II.A. Introduction
Phoretic ink differs from conventional ink in that one “pigment” can present at least two aspects to an appropriately situated observer. This will be referred to as pigment branching. For example, the ability of a phoretic ink pigment to present two aspects will be referred to as two-valued pigment branching. The ability of a phoretic ink pigment to present three aspects will be referred to as three-valued pigment branching. Likewise, phoretic ink that has two-valued pigment branching capability will be referred to as two-valued phoretic ink. Similarly, phoretic ink that has three-valued pigment branching capability will be referred to as three-valued phoretic ink. The mechanics of pigment branching will be discussed in more detail below.
Because conventional ink has a one-to-one correspondence between pigment and color, (i.e., there is no pigment branching in conventional ink) the process of “addressing” conventional ink to produce an image consists in the precise placement of such pigments to specified points on a surface. For example, to “address” a conventional black and white image consisting of text and line drawings on a white surface, black pigment is applied to those points in the image designated as black, and is not applied to those points in the image designated as white. In contrast, and using the example of two-valued phoretic ink where one aspect is white and one aspect is black, the phoretic ink pigment is layered over the entire surface. The entire phoretic ink pigment may then be exposed to a first applied field in order to present a white aspect. Next, only those points on the surface designated as black may be exposed to a second applied field in order to present a black aspect. The method of addressing phoretic ink pigment, thus, is similar to the method of addressing conventional cathode ray tube displays, or conventional liquid crystal displays.
The mechanics of phoretic ink are based on the known phenomena of electrophoresis and magnetophoresis. Electrophoresis refers to the process of applying an electric field to charged elements within a medium such that the charged elements are translationally displaced. The medium is typically a solution and the lowest unit of electric charge is an electric monopole. Thus, translational motion of charged elements through a solution may be achieved by orienting an applied electric field vector parallel to the desired translational vector of the elements.
Magnetophoresis operates similar to electrophoresis with one qualification. That is, magnetophoresis refers to the process of applying a magnetic field to magnetically charged elements within a medium such that the magnetically charged elements are translationally displaced. The lowest unit of magnetic charge, however, is the magnetic dipole. Thus, the orientation of an applied magnetic field vector is not enough to cause translational displacement. Rather, it is the density of magnetic flux lines that determines whether translation occurs. Specifically, if a first region and a second region are situated such that magnetic flux lines converge in the direction of the first region from the second region, then a magnetic dipole will be translationally displaced in the direction of the first region from the second region. This is depicted in FIG. 1 and FIG. 2. In FIG. 1, vector field 32 is oriented in the direction of arrow 36. Also associated with vector field 32 and shown in FIG. 1 is a plurality of flux lines 66. Flux lines 66 are depicted as generally converging in the direction of second region 192 from first region 190. Therefore, the gradient field 46 of vector field 32 is oriented in the direction of arrow 48, which is the same direction as the direction of convergence of flux lines 66.
To further illustrate the significance of vector field versus a gradient field, FIG. 2 depicts vector field 32 oriented in the same direction as shown in FIG. 1. However, in FIG. 2, the flux lines converge in the direction of first region 190 from second region 192. Thus, the gradient field 46 of vector field 32 is in the direction shown by arrow 48. Therefore, even though vector field 32 has the same direction, FIGS. 1 and 2 depict gradient fields in different directions. FIG. 3 depicts the flux lines 66 associated with a magnetic field of a current loop 64, with a current 68 denoted by I. In FIG. 3, the magnetic flux lines 66 converge through the center of current loop 64. Therefore, the situation depicted in FIG. 1 may be achieved by placing current loop 64 coincident with second region 192. Furthermore, the situation depicted in FIG. 2 may be achieved by placing current loop 64 coincident with first region 190. As indicated in FIGS. 1 and 2, a measure of the convergence of magnetic flux lines is given by the gradient of a magnetic field:
 {square root over (∇)}H≠0
Such a field H is an example of a gradient magnetic field. As generally used herein, “vector field” refers to a field whose amplitude in space is capable of having a magnitude and a direction. Vector fields of interest in the present invention include electric fields, magnetic fields, or electromagnetic fields. Furthermore, as used herein, “gradient field” refers to a vector field whose magnitude in a particular displacement direction is not uniform.
An element of two-valued phoretic ink as described, for example, in U.S. Pat. No. 5,930,026, herein incorporated by reference, is depicted in FIG. 4. The two-valued phoretic ink element 10 consists of a microencapsulated set of first aspect medium 14 and second aspect elements 20 within microencapsulation structure 18. The microencapsulation structure 18 can be chosen so as to be transparent to the incident electromagnetic energy of interest 24 and to the transmitted electromagnetic energy of interest 26 to observer 30. In addition, FIG. 4 corresponds to the use of an electric field 34 as the applied field, and corresponds to the use of visible light as the incident electromagnetic energy of interest 24 and the transmitted electromagnetic energy of interest 26 to observer 30. Although FIG. 4 depicts phoretic ink element 10 as spherically symmetric, it will be appreciated by one skilled in the art that phoretic ink element 10 may be of arbitrary shape. The diameter of phoretic ink element 10 may be of the order of magnitude of approximately 10 microns to 400 microns. “Diameter,” as used herein, refers generally to an order of magnitude dimension corresponding to any of height, width, and depth of any microencapsulation structure or aspect elements. The use of “diameter” does not imply that circular or spherical geometry only is under consideration. The second aspect elements 20 in FIG. 4 are depicted as electrically charged particles. One skilled in the art will appreciate, however, that particles alone are not the only options for second aspect element 20. For example, second aspect element 20 could consist, for example, of a liquid drop with a high surface tension.
FIGS. 5 and 6 depict a perspective view of the top of phoretic ink element 10 in the presence of applied vector field 32. The symbol {circle around (x)} indicates an arrow directed into the plane of the figure, and the symbol ⊚ indicates an arrow directed out of the plane of the figure. In FIG. 5, for applied vector field 32 directed into the plane of the figure indicated by arrow 36, observer 30 registers first aspect 16, corresponding to the view of first aspect medium 14. In this situation, all of second aspect elements 20 are translated away from the viewing aspect under the influence of applied field 32. In FIG. 6, for applied field 32 directed out of the plane of the figure indicated by arrow 36, observer 30 registers second aspect 22, corresponding to a view of second aspect elements 20. In this situation, all of second aspect elements 20 are translated towards the viewing aspect under the influence of applied field 32.
II.B. Branching Frequency and Aspect Stability
The branching frequency of a phoretic ink pigment can be defined as the inverse of the time elapsed between the viewing of first aspect 16 and second aspect 22 in the presence of applied field 32, where the applied field 32 may be a vector field or a gradient field. The formula for the branching frequency of electrophoretic ink element 10 of FIG. 4, is thus:   f  =                    V        ⁢                                   ⁢        ɛζ            ⁢                                   6      ⁢      π      ⁢                           ⁢              d        2            ⁢      η      where V is the potential difference associated with the electric field 34, η is the viscosity of first aspect medium 14, ε is the dielectric constant of first aspect medium 14, d is the displacement of second aspect elements 20, and ζ is the Zeta potential of the second aspect elements 20 within first aspect medium 14. The analog to the branching frequency for phoretic ink is the refresh rate for conventional emissive displays. Useful refresh rates for dynamic image viewing are in the range of 60 Hertz or higher. Based upon the above equation, branching frequencies that are in the 60 Hertz or higher range may be made by making d sufficiently small. Exemplary order-of-magnitude values for the above variables are: ζ=600 millivolts; V=200 volts; η=10−4 kilograms/(meter second); and ε=10−13 (kilograms meter)/(second2 volt2).
Another useful property of phoretic ink is the ability to maintain a given aspect after the applied field for addressing is removed. This will be referred to as aspect stability. The mechanism for aspect stability in the above case is generally the energy associated with the attraction between the charged aspect elements 20 and microencapsulation structure 18, or “work function.” A host of factors influence the magnitude of the energy associated with the work function including, but not limited to: surface tension of first aspect medium 14 in contact with second aspect element 20; the relative specific gravity of first aspect medium 14 to second aspect element 20; magnitude of charge on second aspect element 20; relative electronic permittivity of first aspect medium 14 and microencapsulation structure 18; “stickiness” of microencapsulation structure 18; and other residual fields that may be present. The applied field for addressing must be strong enough to overcome the work function in order to cause displacement; furthermore, the work function must be strong enough to maintain this aspect in the absence of an applied field for addressing. FIG. 7 depicts an exemplary diagram of the number of aspect elements displaced 54, N, as a function of applied field 32, V. The work function 52, VW, corresponds to the magnitude of applied vector field 32 when the number of aspect elements displaced 54 has reached the saturation level 56, NS, corresponding to the displacement of all aspect elements 20.
In FIG. 8, an element of two-valued phoretic ink 10 as disclosed, for example, in U.S. Pat. No. 5,411,398, herein incorporated by reference, and that corresponds to the use of a gradient magnetic field 38 is depicted. Again, the microencapsulation structure 18 is transparent to the incident electromagnetic energy of interest 24 and to the transmitted electromagnetic energy of interest 26. The second aspect elements 20 in FIG. 8 may be ferromagnetic particles such as magnetite Fe3O4. Again, one skilled in the art will appreciate that second aspect element 20 is not necessarily a solid. For example, second aspect element 20 in magnetophoretic ink element 10 may be a ferromagnetic liquid with a high surface tension. First aspect medium 14 is a non-magnetic liquid such as a mixture of isoparaffin solvent, titanium oxide, and nonionic surfactant, as disclosed, for example, in U.S. Pat. No. 5,151,032, herein incorporated by reference.
FIGS. 9 and 10 depict a perspective view of the top of phoretic ink element 10 in the presence of applied gradient field 46. Again, the symbol {circle around (x)} indicates an arrow directed into the plane of the figure, and the symbol ⊚ indicates an arrow directed out of the plane of the figure. In FIG. 9, and in the presence of gradient magnetic field 46 directed to the bottom, as indicated by arrow 48, second aspect elements 20 translationally displace to the bottom of phoretic ink element 10 presenting first aspect 16. Conversely, in the presence of gradient magnetic field 46 directed to the top, as indicated by arrow 48 in FIG. 10, second aspect elements 20 translationally displace to the top of phoretic ink element 10 presenting second aspect 22 as indicated in FIG. 10.
The formula for the force exerted by a magnetic field {right arrow over (B)} on a magnetic dipole {right arrow over (m)} is{right arrow over (F)}={right arrow over (∇)}({right arrow over (m)}·{right arrow over (B)})Furthermore, where the gauge of the magnetic field is chosen to satisfy the condition {right arrow over (∇)}×{right arrow over (B)}=0, the above equation has the form:{right arrow over (F)}=({right arrow over (m)}·{right arrow over (∇)}){right arrow over (B)}
The branching frequency in the case of magnetophoretic ink element 10 in FIGS. 9-10 is a function of medium viscosity, the size of the ferromagnetic element, and the magnitude of the gradient magnetic field 46. As disclosed, for example, in U.S. Pat. No. 5,411,398, hereinabove incorporated by reference, the larger the second aspect elements 20, the larger the branching frequency.
The mechanisms responsible for aspect stability in this case are the same as those cited above, with the exception that the second aspect elements 20 are not charged in this case. As has been disclosed in U.S. Pat. No. 4,536,428, herein incorporated by reference, when the specific gravity of second aspect elements 20 is lower than or equal to the specific gravity of the first aspect medium 14, the aspect stability is enhanced but branching frequency drops; contrariwise, when the specific gravity of the second aspect elements 20 is greater than the specific gravity of the first aspect medium 14, the branching frequency is enhanced while the aspect stability deteriorates. Accounting for the competing effects of aspect stability and branching frequency has been an issue in the construction of magnetophoretic ink elements. FIG. 11 depicts an exemplary diagram of the number of aspect elements displaced 54, N, as a function of applied field 46, {right arrow over (∇)}V. The work function 53, {right arrow over (∇)}VW, corresponds to the magnitude of applied vector field 46 when the number of aspect elements displaced 54 has reached the saturation level 57, NS, corresponding to the displacement of all aspect elements 20.
Another issue associated with magnetophoretic ink elements concerns the process of agglomeration. Because each of the second aspect elements 20 acts as a magnetic dipole, there is a tendency for the second aspect elements 20 to attract one another within the microencapsulated structure 18 to form one large element. The process of agglomeration, thus, affects branching frequency, aspect stability, as well as aspect resolution. Much of the work in the area of magnetophoretic ink elements has been the determination of optimal combinations of second aspect element 20, first aspect medium 14, and encapsulating structure 18 in order to balance competing effects, as above.
II.C. Phoretic Ink with More than Two Aspects
Both phoretic ink element 10 depicted in FIGS. 4 and 8 are two-valued phoretic pigments. That is, each pigment can represent, at most, two aspects to a favorable situated observer, as depicted in FIGS. 5-6 and FIGS. 9-10. In order to generalize such a system to include more than two aspects on a macroscopic scale, there have been three options. Each of these options is considered in turn below.
II.C. 1. First Option for Displaying More than Two Aspects
One option is to precisely correlate the position of a set of two-valued phoretic pigments with the addressing mechanism on a sub-pixel level. This is depicted in FIG. 12. There is first two-valued phoretic ink element 72, second two-valued phoretic ink element 74, and third two-valued phoretic ink element 76. A cross section of first two-valued phoretic ink element 72 and applied field 92 is depicted in FIG. 13. Similarly, a cross section of second two-valued phoretic ink element 74 and applied field 94 is depicted in FIG. 14. Further still, a cross section of third two-valued phoretic ink element 76 and applied field 96 is depicted in FIG. 15.
For purposes of illustration, first aspect medium 14 of the two-valued phoretic pigments of FIGS. 13-15 may be chosen so as to present a white-colored aspect to a favorably situated observer; meanwhile, second aspect element 120 of first two-valued phoretic pigment 72 may be chosen so as to present a yellow-colored aspect, third aspect element 130 of second two-valued phoretic pigment 74 may be chosen so as to present a cyan-colored aspect, and fourth aspect element 140 of third two-valued phoretic pigment 76 may be chosen so as to present a magenta-colored aspect. FIG. 12 also depicts first addressing region 82, second addressing region 84, and third addressing region 86. The basis for the addressing in this configuration is the location of the appropriate phoretic pigment within the appropriate addressing region.
In much the same way that conventional ink is addressed, first two-valued phoretic pigment 72, second two-valued phoretic pigment 74, and third two-valued phoretic pigment 76 are spatially applied as to be located only in first addressing region 82, second addressing region 84, and third addressing region 86, respectively. This array is microscopically repeated within each pixel 90. FIG. 12 depicts pixel 90 as triangular in shape. However, one skilled in the art will appreciate that pixel 90 may be in any shape, for example, rectangular. On a macroscopic level, therefore, the first addressing region 82, second addressing region 84, and third addressing region 86 may be manipulated by addressing means (not shown) to produce any three-color image possible.
The advantage of this technique is the ease with which two-valued phoretic inks with different aspects can be created. However, the disadvantage is the precise level of correlation between first addressing region 82, second addressing region 84, and third addressing region 86 and the placement of first two-valued phoretic pigment 72, second two-valued phoretic pigment 74, and third two-valued phoretic pigment 76 that is necessary in order for the display to function. In addition, there is a resulting loss in resolution due to the fact that a given pixel 90 area is at least three times the smallest addressing region, not including the buffer zone necessary to avoid cross-addressing problems.
An even more serious disadvantage of this technique has to do with limitations of reflective (as opposed to emissive displays). If one were to display a cyan color, then the two valued phoretic pigment for cyan would be turned on and all others turned off (white for example). In this mode, only one out of three pixels is reflecting red while two out of three are reflecting white. This leads to a very faint red color and a device of this type is disadvantaged with regards to an emissive display.
II.C.2. Second Option for Displaying More than Two Aspects
A second option for achieving a display with more than two aspects on a macroscopic scale is to mix together in one solution three different pigments, for example. This is depicted in FIG. 16, showing such a mixture applied within pixel 90. In this depiction, addressing region 88 may address any of the first two-valued phoretic pigment 72, second two-valued phoretic pigment 74, or third two-valued phoretic pigment 76.
The manner of addressing this type of composite phoretic ink pigment tends to be more complex. Specifically, one needs to be able to discriminate among the three different types of phoretic ink elements for addressing purposes. If we consider the process of electrophoresis, one manner in which this is accomplished is to use a different magnitude charge on each of the aspect elements. However, any manner of altering the work function associated with each of the phoretic ink elements will work. An exemplary graph of three work functions arranged step-like is depicted in FIG. 17. The lower threshold 102, VWY, represents the threshold necessary to address the saturation number 112, NSY, of the second aspect elements 120; the middle threshold 104, VWC, represents the threshold necessary to address both the saturation number 112, NSy, of second aspect elements 120 and the saturation number 114, NSC, of third aspect elements 130; the highest threshold 106 represents the amount of energy necessary to address all of the saturation number 112, NSY, of second aspect elements 120, the saturation number 114, NSC of third aspect elements 130, and the saturation number 116, NSM, of fourth aspect elements 140.
The process of selectively addressing one of the three elements is, in general, a two-step process. Both third aspect elements 130 and fourth aspect elements 140 require two steps in order to be selectively displaced. The elements that may be addressed in one step only are second aspect elements 120.
For phoretic ink element 72 of FIG. 13 within a composite pigment as depicted in FIG. 16, the magnitude of applied field 92 that will displace second aspect elements 120 is lower than threshold 102 of FIG. 17. As long as the magnitude of the applied field is less than middle threshold 104 of FIG. 17, then neither third aspect elements 130 of FIG. 14 nor fourth aspect elements 140 of FIG. 15 will be displaced to a viewing aspect in FIG. 16.
Proceeding to phoretic ink element 74 of FIG. 14 within a composite pigment as depicted in FIG. 16, the magnitude of applied field 94 that will displace third aspect elements 130 and second aspect elements 120 is middle threshold 104 of FIG. 17. As long as the magnitude of the applied field is less than higher threshold 106 of FIG. 17, then fourth aspect elements 140 of FIG. 15 will not be displaced. In order to cause second aspect elements 120 of FIG. 13 to displace away from a viewing aspect in FIG. 16, a second step is required. The second step consists of directing a second applied field into the plane of FIG. 16 at lower threshold 102 of FIG. 17. Such a second step will leave third aspect elements 130 in a viewing aspect, and will displace second aspect elements 120 away from a viewing aspect.
Finally, for phoretic ink element 76 of FIG. 15 within a composite pigment as depicted in FIG. 16, the magnitude of applied field 96 that will displace fourth aspect elements 140, third aspect elements 130 of FIG. 14, and second aspect elements 120 of FIG. 13 is highest threshold 106 of FIG. 17. Again, in order to cause third aspect elements 130 of FIG. 14 and second aspect elements 120 of FIG. 13 to displace away from a viewing aspect in FIG. 16, a second step is required. The second step consists of directing a second applied field into the plane of FIG. 16 at middle threshold 104 of FIG. 17. Such a second step will leave fourth aspect elements 140 in a viewing aspect, and will displace third aspect elements 130 and second aspect elements 120 away from a viewing aspect.
Such a multi-threshold addressing scheme has been disclosed, for example, in U.S. Pat. No. 5,739,801, herein incorporated by reference. In U.S. Pat. No. 5,739,801, the application was to a twisting ball display; however, the basic problem of selectively addressing display elements that respond to different magnitude vector fields is identical to that of addressing a four-valued phoretic pigment.
The disadvantages associated with this type of four-valued aspect system include the lack of resolution again. Specifically, even though each addressing region 88 may address any of the three aspects, yellow, cyan, or magenta, the statistical distribution of elements (⅓) keeps the resolution low, and thus, the colors are not as richly saturated across the entire display 60 as they would be in the third option considered below.
II.C.3. Third Option for Displaying More than Two Aspects
The final option available in order to produce more than a two-aspect display on a macroscopic scale is to create higher-valued phoretic ink, such as four-valued phoretic ink. An element of four-valued phoretic ink is depicted in FIG. 18. Suitable examples are disclosed, for example, in U.S. Pat. No. 6,017,584, herein incorporated by reference. In FIG. 18, the applied field used for addressing is an electric field. Within four-valued phoretic ink element 78 is first aspect medium 14, second aspect elements 120, and third aspect elements 130, and fourth aspect elements 140. The addressing means for this type of phoretic ink element is the same type of multi-threshold scheme described above. Again, second aspect element 120, third aspect element 130, and fourth aspect element 140 are all discriminated upon, for addressing purposes, by some manner of increasing the work function for each one individually. For instance, in the exemplary four-valued phoretic ink element depicted in FIG. 18, it can be the magnitude of the charge that allows for addressing discrimination between the three aspect elements.
Again, the process of selectively addressing one of the three elements is, in general, a two-step process. Both third aspect elements 130 and fourth aspect elements 140 of FIG. 18 require two steps in order to be selectively displaced. The elements that may be addressed in one step only are second aspect elements 120.
As before, an exemplary graph of three work functions arranged step-like is depicted in FIG. 17. The lower threshold 102, VWY, represents the threshold necessary to address the saturation number 112, NSY, of the second aspect elements 120; the middle threshold 104, VWC, represents the threshold necessary to address both the saturation number 112, NVSY, of second aspect elements 120 and the saturation number 114, NSC, of third aspect elements 130; the highest threshold 106 represents the amount of energy necessary to address all of the saturation number 112, NSY, of second aspect elements 120, the saturation number 114, NSC of third aspect elements 130, and the saturation number 116, NSM, of fourth aspect elements 140.
For phoretic ink element 78 of FIG. 18, the magnitude of applied field 92 will displace second aspect elements 120 when it is at lower threshold 102 of FIG. 17. As long as the magnitude of the applied field is less than middle threshold 104 of FIG. 17, then neither third aspect,elements 130 nor fourth aspect elements 140 will be displaced to the top of phoretic ink element 78.
The magnitude of applied field 92 will displace third aspect elements 130 and second aspect elements 120 of FIG. 18 when it is at middle threshold 104 of FIG. 17. As long as the magnitude of the applied field is less than higher threshold 106 of FIG. 17, then fourth aspect elements 140 will not be displaced. In order to cause second aspect elements 120 to displace away from the top of phoretic ink element 78, a second step is required. The second step consists of directing a second applied field towards the bottom of phoretic ink element 78 at lower threshold 102 of FIG. 17. Such a second step will leave third aspect elements 130 at the top of phoretic ink element 78, and will displace second aspect elements 120 towards the bottom of phoretic ink element 78.
Finally, the magnitude of applied field 92 in FIG. 18 will displace fourth aspect elements 140, third aspect elements 130, and second aspect elements 120 when it is at highest threshold 106 of FIG. 17. Again, in order to cause third aspect elements 130 and second aspect elements 120 to displace away from the top of phoretic ink element 78, a second step is required. The second step consists of directing a second applied field towards the bottom of phoretic ink element 78 at middle threshold 104 of FIG. 17. Such a second step will leave fourth aspect elements 140 at the top of phoretic ink element 78, and will displace third aspect elements 130 and second aspect elements 120 towards the bottom of phoretic ink element 78.
A display 60 based upon the use of phoretic ink element 78 is depicted in FIG. 19. Phoretic ink elements 78 depicted in pixel 90 are addressed in FIG. 19 to present the aspect associated with fourth aspect elements 140. Unlike the previously described displays, display 60 of FIG. 19 using phoretic ink elements 78 is capable of presenting a saturated aspect to favorably situated observer 30.
One skilled in the art will appreciate that phoretic ink element 78, from a macroscopic perspective, may present more than four aspects, since there is an admixture of aspect elements that it is also possible to address. For example, by applying the middle threshold, both the second aspect elements 120 and the third aspect elements 130 are translationally displaced to the top of the phoretic ink element 78. If second aspect elements 120 are yellow-colored, and third aspect elements are cyan-colored, then applying the middle threshold will produce a green-colored aspect to a favorable situated observer. Thus, in addition to presenting a cyan-colored aspect, a yellow-colored aspect, and a magenta-colored aspect, phoretic ink element 78 may present a green-colored aspect to observer 30. Other combinations are also possible.
In light of the foregoing, it remains desirable to fabricate a phoretic ink element that exhibits three-valued pigment branching, or higher-valued pigment branching, and that incorporates a relatively simple addressing scheme in order to produce a rich hue.